Wasm initial `Window::inner_size()`

[daxpedda]

After Window creation, one has to wait for a Resized event, otherwise Window::inner_size() will always return None.

I don't know what the best solution is here, but it might be better to split the test function into two parts: Window creation and testing. It looks to me like other backends encounter similar problems; e.g. in Softbuffer you have to wait until the window is mapped.